Add maglev_backend_info variable that correlates backend/address/healthcheck/source_tag

internal/metrics/metrics.go

@@ -21,9 +21,10 @@ import (
 
 // BackendInfo holds the health and config state needed by the collector.
 type BackendInfo struct {
-	Health  *health.Backend
-	Enabled bool
-	HCName  string // healthcheck name from config
+	Health    *health.Backend
+	Enabled   bool
+	HCName    string // healthcheck name from config
+	SourceTag string // nginx source tag; equals backend name when unset in config
 }
 
 // StateSource provides read-only access to the running checker state.
@@ -134,6 +135,7 @@ type Collector struct {
 	src StateSource
 	vpp VPPSource // optional; nil when VPP integration is disabled
 
+	backendInfo    *prometheus.Desc
 	backendState   *prometheus.Desc
 	backendHealth  *prometheus.Desc
 	backendEnabled *prometheus.Desc
@@ -156,6 +158,11 @@ func NewCollector(src StateSource, vpp VPPSource) *Collector {
 	return &Collector{
 		src: src,
 		vpp: vpp,
+		backendInfo: prometheus.NewDesc(
+			"maglev_backend_info",
+			"Static backend metadata. Always 1; metadata is conveyed via labels.",
+			[]string{"backend", "address", "healthcheck", "source_tag"}, nil,
+		),
 		backendState: prometheus.NewDesc(
 			"maglev_backend_state",
 			"Current backend state (1 = active for the given state label).",
@@ -216,6 +223,7 @@ func NewCollector(src StateSource, vpp VPPSource) *Collector {
 
 // Describe implements prometheus.Collector.
 func (c *Collector) Describe(ch chan<- *prometheus.Desc) {
+	ch <- c.backendInfo
 	ch <- c.backendState
 	ch <- c.backendHealth
 	ch <- c.backendEnabled
@@ -247,6 +255,11 @@ func (c *Collector) Collect(ch chan<- prometheus.Metric) {
 		}
 		addr := info.Health.Address.String()
 
+		ch <- prometheus.MustNewConstMetric(
+			c.backendInfo, prometheus.GaugeValue, 1.0,
+			name, addr, info.HCName, info.SourceTag,
+		)
+
 		// One time-series per possible state; the current state is 1, rest 0.
 		for _, s := range states {
 			val := 0.0